Device for detecting faults in an apparatus for detecting knocking

ABSTRACT

A device for detecting faults in conjunction with the detection of knocking in an internal combustion engine is disclosed, in which device the sensor output signals are compared with an engine speed-dependent, normalized reference level in order to detect knocking. As a function of the comparison result, knocking is detected and, when knocking is detected, knocking-preventing measures are initiated. So that the internal combustion engine does not erroneously go into unacceptable operating states when a knocking sensor or the associated evaluation circuit is operating incorrectly, continuous detection of faults takes place, during which it is tested whether the normalized reference level lies within a permitted range which is formed as a function of engine speed, this range being either the permitted, normalized reference level band or a range between an engine speed-dependent upper limit value (UGO) and an engine speed-dependent lower limit value (UGU). If the normalized reference signal level leaves this range for a prescribable time, a fault is detected and safety measures are initiated which are effective until the reference level lies within the permitted range again.

PRIOR ART

The invention is based on a device for detecting faults, of the generictype of the main claim.

It is known that, in internal combustion engines with knocking control,it is necessary to monitor the operational capability of the knockingsensor which makes detection of knocking possible. Otherwise, there isthe risk that in the event of the knocking sensor or a component of theassociated evaluation circuit failing knocking is not detected and theworking point of the internal combustion engine is erroneously shiftedinto the knocking range, which can result in a hazard for the internalcombustion engine itself.

Therefore, in DE-PS 31 28 475 a testing device for a knocking controlapparatus on an internal combustion engine is proposed, in which testingdevice a knocking sensor with an evaluation circuit which is connecteddownstream and triggers shifting of the ignition angle when knockingoccurs is tested for correct operational capability.

In the known device, the knocking detection takes place during ameasuring phase. For this purpose, the output signal of the knockingsensor is compared in a known manner with an adaptable reference value.Knocking is detected from the result of the comparison. During a testphase, the output signal of the knocking sensor is compared withanother, lower reference value. Operational faults are then detectedfrom this comparison result if the output signal does not exceed thereference signal.

The known testing device for a knocking control apparatus on an internalcombustion engine has the disadvantage that a measuring phase and a testphase are required and testing is not possible in the measuring phase,while in the test phase detection of knocking cannot be carried out.

ADVANTAGES OF THE INVENTION

The device according to,the invention for detecting faults in anapparatus for detecting knocking in an internal combustion engine has,in contrast, the advantage that the detection of knocking and thedetection of faults can proceed alongside one another withoutinterruption so that the occurrence of knocking can be detected at anytime and also the detection of faults can be carried out withoutinterruption.

These advantages are achieved in that the reference level, the exceedingof which is the criterion for the detection of knocking, is initiallynormalized in such a way that it rises as the engine noise increases.Here, this reference level is, on the one hand, used for the detectionof knocking and, on the other hand, a comparison is made as to whetherthe normalized reference level lies within a permitted reference levelband. The permitted reference level band is specified in a suitablemanner, in particular as a function of the engine speed. A fault isdetected if the normalized reference level leaves the permittedreference level band. Here, it is advantageous that detection of faultscan only occur if the normalized reference level not only leaves therange permitted for the normalized reference level but also exceeds anupper limit value which is dependent on the engine speed or drops belowa lower limit value which is also dependent on engine speed.

Further advantages of the invention are achieved with the measuresdisclosed in the subclaims. Here, an advantageous feature for increasingthe reliability of the detection of faults is that a fault is onlydetected if the normalized reference level lies outside the permittedreference level band for a specific, selectable time.

It is particularly advantageous that after detection of a fault safetymeasures are automatically initiated which ensure that the internalcombustion engine is always operated in a non-hazardous range. For thispurpose, the detection of faults is followed for example by an ignitionangle adjustment which is maintained at least until it is detected thatthe apparatus for detecting knocking is functioning again correctly.

So that the safety measures initiated after detection of a fault are notterminated too early, in a further advantageous embodiment of theinvention the cancelling of the safety measures is not initiated untilthe instantaneous reference level remains within the reference bandagain during a prescribable time.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows the design of an exemplary embodiment in terms ofcircuitry.

FIG. 2 shows the interrelation between reference level and engine speed.

FIG. 3 is a flow diagram which illustrates the procedure for thedetection of faults.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

In the exemplary embodiment illustrated in FIG. 1, an apparatus fordetecting knocking in an internal combustion engine with two knockingsensors 10, 11 is illustrated. These knocking sensors 10, 11 areconnected via an evaluation circuit 12 to the input E1 of amicrocomputer 13 which is a component of a control unit (not illustratedin greater detail).

Further signals which for example characterize the operating state ofthe internal combustion engine can be fed to the microcomputer 13 via aninput E2. Such a signal can be the engine speed n of the internalcombustion engine which is measured with a sensor. As a function of thesignals of the evaluation circuit 12 and of the further signals whichare fed to the input E2, the microcomputer 13 controls the ignitionoutput stage 14 of the internal combustion engine and/or, ifappropriate, further devices of the internal combustion engine, forexample by emitting appropriate signals at the output A.

The microcomputer 13 can also be designed as a discrete circuit, and theevaluation circuit 12 and the microcomputer 13 can equally be combinedto form a single computing device and its operation can be implementedfor example with the aid of the control unit of the internal combustionengine.

Instead of, or as an alternative to, influencing the ignition time pointas a measure for the displacement of the working point of the internalcombustion engine out of the knocking range, the other measures, knownper se, for influencing the knocking behavior of an internal combustionengine, for example the enrichment of the combustion mixture, theaddition of means for achieving higher resistance to knocking etc., canbe used.

In the selected exemplary embodiment, the evaluation circuit 12comprises at least one amplifier 15 with adjustable amplificationfactor, to which the output signals which are supplied by the knockingsensors 10, 11 are fed alternately. In an adjoining bandpass filter 16,the amplified signals are filtered in such a way that the signalcomponents with frequencies which are typical of knocking are preferred.

A demodulation circuit 17, for example a rectifier, adjoins the bandpassfilter. The signals which are emitted by the demodulation circuit areintegrated in the integrator 18, and the integrated signals KI arepassed on to a first input of the comparator 19.

The other input of the comparator 19 is fed a reference signal or areference level Ref which is formed for example in the lowpass filter 22by averaging the knocking sensor signals using a large time constant.The actual level of the reference signal is however influenced using themicrocomputer 13 as a function of the operating states of the internalcombustion engine. It is equally possible to influence the level of theoutput signals of the knocking sensors 10, 11 directly in that thecontrol amplifier 15 is actuated by the microcomputer 13 in a suitableway.

The precise design of the individual elements is not significant forcomprehension of the invention and is therefore not indicated in greaterdetail. Likewise, there is only symbolic representation of the internalcombustion engine 21 to which the knocking sensors 10 and 11 areassigned.

Using the apparatus illustrated in FIG. 1 the detection of knockingproceeds as follows: The knocking sensors 10, 11 record the noises whichare made by the internal combustion engine 21 and output appropriatesignals S1, S2 to the evaluation circuit 12. These signals are filteredin a suitable way and amplified in the control amplifier 15.

After conditioning in the bandpass filter 16, the demodulator circuit 17and the integrator 18, the output signals of this control amplifier 15are compared with a reference level Ref in the comparator 19. At theoutput of the comparator 15 a signal S3 is produced which permits thepresence or absence of knocking to be detected, in which processknocking is detected if the output signal of the integrator 18 exceedsthe reference level in a prescribable way.

One possible way of forming a particularly suitable reference signal, orthe reference level Ref, will now be explained. For this, the referencelevel Ref is fixed or normalized in the microcomputer 13 in such a waythat a continuous rise in the normalized reference level from quietnoise to loud noise of the internal combustion engine is produced.

The determination of the instantaneous reference level Ref is carriedout for example according to the formula:

    Ref=(F1-1)/F1 Refa+1/F1 KI

    Ref=15/16Refa+1/16KI

In this formula:

Ref=reference level,

Refa=previous reference level,

KI=knocking integral,

F1=16=adjustable factor.

In order to form the normalized reference level, for example thereference level is multiplied with a normalizing factor V(i), a possibleformula for forming the normalized reference level is:

    Refn(i)=(8/V(i)) * Ref(i)

In which:

Refn=normalized reference level,

Ref=reference level,

V(i)=normalizing factor for normalized reference level.

Normalization can take place by adjusting the amplification level as afunction of the amplification factor. The normalizing factor has e.g.values of V(i)=1, 2, 4, 8, 16, 32, 64.

In order to detect incorrect operation of the knocking sensors or afault in the evaluation circuit, a permitted reference level band RefBis defined with respect to the engine speed n, as is illustrated in FIG.2. Furthermore, lower and upper limit values UGU, UGO, which lie at asuitable distance from the limits of the permitted reference level band,are defined as a function of the engine speed. Here, the distancebetween the limits is specified empirically, a vehicle-specificadaptation can also take place.

In the microcomputer 13, it is tested continuously whether therespectively determined, instantaneous, normalized reference levelRefn(i) drops below or exceeds the lower or upper limit value. If thisis the case, in the simplest case errors are detected and suitablecounter measures, such as the adjustment of the ignition or enrichmentof the mixture, are initiated by the microcomputer. The initiated safetymeasures are cancelled again in the case of this detection of a fault assoon as the instantaneous reference level lies within the referencelevel band again.

In order to increase the reliability of the detection of faults, furthertests and measures are possible, which, if they are not fulfilled,prevent a defect from being detected. One of these measures consists inthe fact that the detection of faults is not carried out until theengine speed of the internal combustion engine lies above a limit speednG.

A further limitation consists in the fact that, in order to detectfaults, it is tested whether the normalized reference signal Refn(i)lies outside the permitted reference band for a specific time period t1,and the safety measures are only initiated when this condition isfulfilled.

Furthermore, it is possible for the initiated safety measures not to becancelled until the instantaneous reference level again lies within thereference level band RefB for a further prescribable time period t2.

In a further embodiment, the safety measures, such as the adjustment ofthe ignition in the late direction, are not initiated until it isestablished by the microcomputer 13 that the knocking control is active.However, the detection of faults can also take place when this conditionis not fulfilled.

In an arrangement with a plurality of knocking sensors, the knockingsensor which is detected as defective can be excluded from theevaluation and the knocking control can be carried out using only theknocking sensor which is detected as operationally reliable.

In a further embodiment, the effects of the knocking control areeliminated but the knocking control remains active in order to overcomefaults; also, the instantaneous adjustments in the late direction can becleared from the memory as a function of the fault state detected andthe adjustment in the late direction can be set to zero and the knockingcontrol adaptation is then idle.

In FIG. 2, permitted reference signal levels are entered against theengine speed n. It is clear here that the permitted reference signallevel range RefB becomes larger as the engine speed rises. Here, boththe minimum permitted reference level and the maximum permittedreference level rise as the engine speed rises. Overall, this rise ishowever such that the permitted range RefB increases as the engine speedrises.

Above and below the permitted range further limits UGU, OGO arespecified which change with a corresponding dependence on the enginespeed and are stored in memories of the microcomputer 13 as engine-speeddependent characteristic curves. If the permitted reference signal levelexceeds the upper limit or if it drops below the lower limit, a fault isdetected, if appropriate specific time criteria also being taken intoaccount, as already explained.

The value nG is entered in FIG. 2 as the limit engine speed above whichfaults begin to be detected.

In FIG. 3, an example is given which illustrates a possible procedurefor the detection of faults. The detection of faults described in FIG. 3usually proceeds in the control unit of the internal combustion enginein which the required computing, storage and counting means are present.Here, in a first step s1 the control unit detects from data fed to itfrom individual sensors, for example from the signals of an engine speedsensor, of an airflow rate meter and/or throttle valve signaltransmitter, whether the load dynamic or engine speed dynamic is active.If this is the case, an instantaneous reference level is formed in steps2 according to the formula Ref=3/4Refa+1/4KI. If, on the other hand, itis detected in step s1 that there is no load dynamic or engine speeddynamic present, the instantaneous reference level is formed in step s3according to the equation Ref=15/16Refa+1/16KI.

Starting from these values for the instantaneous reference level, in thefollowing steps it is tested whether the normalized reference level Refnis lower than UGO or it is tested whether the normalized reference levelis higher than a lower limit value UGU.

For this purpose, in step s4, the normalized reference level isinitially formed according to the formula Refn=(8/VFAK)×Ref. In step s5,the normalized reference level is formed according to the formula(32/VAFK)×Ref.

In steps s6 and s7 it is tested whether the engine speed n is higherthan the limit engine speed nG and it is tested whether the enginetemperature Tmot is higher than a critical value TNKR. The steps s6 ands7 are identical, but they are based on the normalized reference levelsdetermined in steps s4 or s5.

If it is detected in steps s6, s7 that the conditions are not fulfilled,the program beings again with step s1. If, on the other hand, it isdetected that the conditions are fulfilled, the program goes to therespective next step s8 or s9. The condition that the engine temperaturemust lie above a critical value can also be dispensed with in step s6and s7.

In step s8 it is tested whether the normalized reference level Refn islower than the upper limit value UGO. In step s9 it is tested whetherthe normalized reference level according to step s7 is higher than thelower limit value UGU. If the condition of the step s8 or the conditionof the step s9 is fulfilled, in step s10 a fault counter is decremented,that is to say counted down by 1. In step s10, it is tested whether thefault counter has already reached the value 0, if this is not the casethe program begins again with step s1, if, on the other hand, the faultcounter is at 0, alternative measures are activated in step s12.

If it is detected in step s11 that the fault counter is not equal to 0,the program for the next cylinder begins, this is symbolized by steps13. If it is detected in steps s8 and s9 that the normalized referencevalue Refn is lower than the upper limit value UGO or higher than thelower limit value UGU, in step s14 a fault elimination counter isdecremented or a fault counter is reset. In step s15 it is then testedwhether the counting value of the fault elimination counter is equal to0. If this is not the case, the program begins with step s1 again.

If, on the other hand, the fault elimination counter is at 0, thealternative measures are rescinded, the rescinding not being carried outuntil the knocking control has been in an inactive phase at least once.The safety measures are therefore cancelled again if the instantaneousreference level lies within the range prescribed by the referencevoltage thresholds UGO and UGU. So that no jumps in the ignition angle(torque) occur during the elimination of faults, the transition from asafety adjustment in the late direction of the ignition angle to normaloperation does not take place until the first inactive phase of knockingcontrol after it has been established that faults are being eliminated.The values which are stored in the fault counters are stored permanentlyso that they remain stored even when the internal combustion engine isswitched off.

In a fault management system or fault storage management system, theconditions for the initiation of alternative measures can be specifiedusing OR gates and RS flipflops. Likewise, it is possible to detect whena test cycle for a sensor has been completely executed.

We claim:
 1. A device for detecting faults in an apparatus for detectingknocking in an internal combustion engine, having at least one knockingsensor and one evaluation circuit which is connected downstream of thelatter and which compares the output signals of the knocking sensor witha variable reference level which is formed as a function of the outputsignals of the knocking sensor, and detects knocking as a function ofthe comparison result, wherein the reference level (Ref) is normalizedin such a way that it rises as the noise of the internal combustionengine becomes greater, wherein a permitted reference level band (RefB)is formed and a fault is detected if an instantaneous, normalizedreference level value (Refn(i)) lies outside the permitted referencelevel band (RefB).
 2. The device as claimed in claim 1, wherein a faultis only detected if a plurality of normalized reference level values(Refn(i)) lie outside the permitted reference level band and/or if thenormalized reference level lies outside the permitted reference levelband (RefB) for a prescribable time period (t1).
 3. The device asclaimed in claim 1, wherein detection of faults does not take placeuntil the normalized reference level value (Refn(i)) lies above afurther engine speed-dependent limit value (UGO) or below a furtherengine speed-dependent limit value (UGU).
 4. The device as claimed inclaim 3, wherein these two limit values are formed as a function of thepermitted reference level band (RefB).
 5. The device as claimed in claim1, wherein the permitted reference level band (RefB) is specified as afunction of the engine speed (n) and rises overall and widens as theengine speed rises.
 6. The device as claimed in claim 1, wherein a lowerengine speed limit value (nG) is specified and detection of faults isonly performed at engine speeds above this limit value (nG).
 7. Thedevice as claimed in claim 1, wherein detection of faults is onlycarried out when the knocking control is active.
 8. The device asclaimed in claim 1, wherein measures are initiated when a fault isdetected, in particular the ignition angle is adjusted and/or the fuelmixture is made richer, these measures being devised such that theinternal combustion engine is in a non-hazardous state.
 9. The device asclaimed in claim 7, wherein the measures are terminated again when thereference level is inside the permitted range again during aprescribable time period (t2).